Typically, a woman delivers birth after nine months. After 42 days, an ostrich hatchling emerges from its egg. Yet, how long did it take for a dinosaur egg to hatch?

A photograph of a Protoceratops andrewsi hatchling fossil discovered in the Gobi Desert Ukhaa Tolgod, Mongolia. [Photograph by: AMNH/M. Ellison]

Depending on the dinosaur, groundbreaking study done by a researcher at Florida State University provides a timetable ranging from three to six months.

In a paper published in the Proceedings of the National Academy of Sciences, FSU Professor of Biological Science Gregory Erickson and a team of researchers describe how embryonic dental records solved the enigma of dinosaur egg incubation duration.

The embryology of dinosaurs is shrouded in mystery; essentially little is known, according to Erickson. “Did their eggs hatch slowly, like those of their reptile relatives, crocodilians and lizards? Or swiftly like living dinosaurs – birds?”

Long ago, scientists hypothesized that dinosaur incubation length was comparable to that of birds, whose eggs hatch between 11-85 days. Comparable-sized reptile eggs might take anything from weeks to months to hatch.

Researchers examined a fossilized embryo of the dinosaur Hypacrosaurus 
[Credit: Darla Zelinitsky]

Because the eggs of dinosaurs were so large — some were about 4 kilograms or the size of a volleyball — scientists believed they must have experienced rapid incubation with birds inheriting that characteristic from their dinosaur ancestors.

Erickson, FSU graduate student David Kay and colleagues from University of Calgary and the American Museum of Natural History decided to put these theories to the test.

To do that, they accessed some rare fossils — those of dinosaur embryos.

“Time within the egg is a crucial part of development, but this earliest growth stage is poorly known because dinosaur embryos are rare,” said co-author Darla Zelenitsky, assistant professor of geoscience at University of Calgary. “Embryos can potentially tell us how dinosaurs developed and grew very early on in life and if they are more similar to birds or reptiles in these respects.”

Protoceratops nest [Credit: FSU]

The two types of dinosaur embryos researchers examined were those from Protoceratops — a sheep-sized dinosaur found in the Mongolian Gobi Desert whose eggs were quite small (194 grams) — and Hypacrosaurus, an enormous duck-billed dinosaur found in Alberta, Canada with eggs weighing more than 4 kilograms.

Erickson and his team ran the embryonic jaws through a CT scanner to visualize the forming dentition. Then, they extracted several of the teeth to further examine them under sophisticated microscopes.

Researchers found what they were looking for on those microscope slides. Growth lines on the teeth showed researchers precisely how long the dinosaurs had been growing in the eggs.

“These are the lines that are laid down when any animal’s teeth develops,” Erickson said. “They’re kind of like tree rings, but they’re put down daily. We could literally count them to see how long each dinosaur had been developing.”

Gregory Erickson is a professor of biological science at Florida State University 
[Credit: FSU]

Their results showed nearly three months for the tiny Protoceratops embryos and six months for those from the giant Hypacrosaurus.

“Dinosaur embryos are some of the best fossils in the world,” said Mark Norell, Macaulay Curator for the American Museum of Natural History and a co-author on the study. “Here, we used spectacular fossils specimens collected by American Museum expeditions to the Gobi Desert, coupled them with new technology and new ideas, leading us to discover something truly novel about dinosaurs.”

The implications of long dinosaur incubation are considerable.

In addition to finding that dinosaur incubation was similar to primitive reptiles, the researchers could infer many aspects of dinosaurian biology from the results.

Daily growth lines in the dentine of an embryonic tooth of Hypacrosaurus 
[Credit: © G.M. Erickson]

Prolonged incubation put eggs and their parents at risk from predators, starvation and other environmental risk factors. And theories that some dinosaurs nested in the more temperate lower latitude of Canada and then traveled to the Arctic during the summer now seem unlikely given the time frame for hatching and migration.

The biggest ramification from the study, however, relates to the extinction of dinosaurs. Given that these warm-blooded creatures required considerable resources to reach adult size, took more than a year to mature and had slow incubation times, they would have been at a distinct disadvantage compared to other animals that survived the extinction event.

“We suspect our findings have implications for understanding why dinosaurs went extinct at the end of the Cretaceous period, whereas amphibians, birds, mammals and other reptiles made it through and prospered,” Erickson said.

Source: Florida State University [January 02, 2017]